As demands for further enhancement of image definition on a display screen are becoming more strict along with a growing need to reduce external light reflectivity thereon for prevention of unwanted on-screen visualization of xe2x80x9cghostxe2x80x9d images of exterior scenes, color picture tubes with a panel having a flat outer surface have been developed in recent years.
One prior known approach to achieving this is to let the panel have a certain curvature on its inner surface while making the panel""s outer face flat. This methodology has the advantage of allowing a shadow mask to exhibit a curved plane, which in turn makes it possible to take full advantage of currently available shadow-mask manufacturing architectures. On the other hand, the prior known approach suffers from the need to specifically design the panel in such a way as to provide an excessively large curvature on the inner panel face as compared to that on the outer face thereof, which would result in an excessive increase in glass plate thickness at or near the periphery of the panel when compared to the glass thickness at the center thereof This disadvantageously creates a serious problem. For example, possible differences in brightness or luminance between certain locations, one of which is at the center of a phosphor film and the other of which is the periphery of the phosphor plane, along with influence or interference of the curved plane on the inner panel face, can deteriorate or degrade the flatness of the display screen as it appears to the users. Accordingly, in the event that the outer panel face is made flat, it seems desirable for the curvature of the inner panel face to be as small in value as possible.
On the other hand, the manufacturability increases when the shadow mask has a larger curvature. A traditionally proposed way of solving this is that the between-electron-beams distance S-size is designed so that it is made smaller at the periphery of the display screen than at the center thereof This technique has been disclosed, for example, in a report of the International Display Workshop (IDW) ""98 at pp. 412-416. This document discloses a technique for reducing the net value of the S-size at the periphery by employing a couple of electromagnetic quadrupoles. Unfortunately, the advantage of this approach does not come without accompanying problems. For example, those images which are visually displayed on the screen have less contrast; additional panel-surface processing will be required inevitably; and, the influence of geomagnetism becomes greater due to the fact that the resulting distance between the inner panel face and its associative shadow mask, i.e. the q-size, remains greater at the periphery.
The object of the present invention is to provide a color picture tube including a panel section having an outer surface made substantially flat and which exhibits required contrast and geomagnetic margins or the like without requiring use of panel-surface processing even in cases where a press formed shadow mask is used. To attain this object, the present invention as disclosed herein uses tint or dark tint materials as a glass material for the panel, while at the same time specifically setting the difference between a glass plate thickness, Tc, at the center of such glass and the glass thickness, Td, at the periphery thereof, namely, a ratio Wd/Tc of the so-called xe2x80x9cwedge amountxe2x80x9d Wd to the value Tc, to be less than or equal to 0.8, or alternatively, by letting the absolute value of Wd be set at 12 millimeters (mm) or less. Further, letting the S-size at the cathodes of an electron gun be set at 6.0 mm or below makes it possible to suppress or minimize any possible influence or interference of the geomagnetism at the periphery of a display screen. While two pairs of electromagnetic quadrupole coils are used to render the net value of such S-size smaller at the screen periphery than in the center thereof, the first one of such electromagnetic quadrupole coils is provided so as to overlie the neck portion of the picture tube along with more than one ring-shaped purity/convergence magnet (PCM) operatively associated therewith.